Clear out empty event_push_summary rows

[matrixbot]

This issue has been migrated from #13875.

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After #13776 we generate a row in event_push_summary per (user, room, thread). If a receipt marks a thread as completely read then the summary is (somewhat) useless as the notif_count and unread_count would be 0.

It might make sense to clean these up eventually to avoid the table growing too large.

There's some conversation about this at matrix-org/synapse#13776 (comment).